Poly(azine)-based charge transport materials

ABSTRACT

An improved organophotoreceptor comprises an electrically conductive substrate and a photoconductive element on the electrically conductive substrate, the photoconductive element comprising:
         (a) a charge transport material having the formula       

     
       
         
         
             
             
         
       
         
         
           
             where X 1  and X 2  are, each independently, a linking group; 
             Ar comprises an aromatic group; 
             R 1 , R 2 , and R 3  comprise, each independently, H, an alkyl group, an alkenyl group, an alkynyl group, an aromatic group, or a heterocyclic group; and 
             n is a distribution of integers between 1 and 100,000 with an average value of greater than one; and 
             (b) a charge generating compound. 
             Corresponding electrophotographic apparatuses, imaging methods, and methods of forming the charge transport material are described.

FIELD OF THE INVENTION

This invention relates to organophotoreceptors suitable for use in electrophotography and, more specifically, to organophotoreceptors having a charge transport materials comprising a polymer having repeating aromatic azine groups. Furthermore, the invention further relates to methods for forming a charge transport material comprising a polymer having repeating aromatic azine groups.

BACKGROUND OF THE INVENTION

In electrophotography, an organophotoreceptor in the form of a plate, disk, sheet, belt, drum or the like having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive layer, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas where light strikes the surface, thereby forming a pattern of charged and uncharged areas, referred to as a latent image. A liquid or solid toner is then provided in the vicinity of the latent image, and toner droplets or particles deposit in the vicinity of either the charged or uncharged areas to create a toned image on the surface of the photoconductive layer. The resulting toned image can be transferred to a suitable ultimate or intermediate receiving surface, such as paper, or the photoconductive layer can operate as an ultimate receptor for the image. The imaging process can be repeated many times to complete a single image, for example, by overlaying images of distinct color components or effect shadow images, such as overlaying images of distinct colors to form a full color final image, and/or to reproduce additional images.

Both single layer and multilayer photoconductive elements have been used. In single layer embodiments, a charge transport material and charge generating material are combined with a polymeric binder and then deposited on the electrically conductive substrate. In multilayer embodiments, the charge transport material and charge generating material are present in the element in separate layers, each of which can optionally be combined with a polymeric binder, deposited on the electrically conductive substrate. Two arrangements are possible for a two-layer photoconductive element. In one two-layer arrangement (the “dual layer” arrangement), the charge-generating layer is deposited on the electrically conductive substrate and the charge transport layer is deposited on top of the charge generating layer. In an alternate two-layer arrangement (the “inverted dual layer” arrangement), the order of the charge transport layer and charge generating layer is reversed.

In both the single and multilayer photoconductive elements, the purpose of the charge generating material is to generate charge carriers (i.e., holes and/or electrons) upon exposure to light. The purpose of the charge transport material is to accept at least one type of these charge carriers and transport them through the charge transport layer in order to facilitate discharge of a surface charge on the photoconductive element. The charge transport material can be a charge transport compound, an electron transport compound, or a combination of both. When a charge transport compound is used, the charge transport compound accepts the hole carriers and transports them through the layer with the charge transport compound. When an electron transport compound is used, the electron transport compound accepts the electron carriers and transports them through the layer with the electron transport compound.

Organophotoreceptors may be used for both dry and liquid electrophotography. There are many differences between dry and liquid electrophotography. A significant difference is that a dry toner is used in dry electrophotography, whereas a liquid toner is used in liquid electrophotography. A potential advantage of liquid electrophotography is that it can provide a higher resolution and thus sharper images than dry electrophotography because liquid toner particles can be generally significantly smaller than dry toner particles. As a result of their smaller size, liquid toners are able to provide images of higher optical density than dry toners.

In liquid electrophotography, the organophotoreceptor is in contact with the liquid carrier of a liquid toner while the toner dries or pending transfer to a receiving surface. As a result, the charge transport material in the photoconductive element may be removed by extraction by the liquid carrier. Over a long period of operation, the amount of the charge transport material removed by extraction may be significant and, therefore, detrimental to the performance of the organophotoreceptor.

SUMMARY OF THE INVENTION

This invention provides organophotoreceptors having good electrostatic properties such as high V_(acc) and low V_(dis). This invention also provides organophotoreceptors comprising a charge transport material having reduced extraction by liquid carriers.

In a first aspect, an organophotoreceptor comprises an electrically conductive substrate and a photoconductive element on the electrically conductive substrate, the photoconductive element comprising:

(a) a charge transport material comprising a polymer having the formula:

where X₁ and X₂ are, each independently, a linking group, such as —(CH₂)_(m)— group, where m is an integer between 1 and 20, inclusive, and one or more of the methylene groups is optionally replaced by O, S, N, C, B, Si, P, C═O, O═S═O, an NR_(a) group, a CR_(b) group, a CR_(c)R_(d) group, or a SiR_(e)R_(f) where R_(a), R_(b), R_(c), R_(d), R_(e), and R_(f) are, each independently, a bond, H, a hydroxyl group, a thiol group, a carboxyl group, an amino group, an alkyl group, an alkoxy group, an alkenyl group, an alkynyl group, a heterocyclic group, an aromatic group, or a part of a ring group, such as cycloalkyl groups or a benzo group;

Ar comprises an aromatic group, such as an aromatic C₆H₃ group;

R₁, R₂, and R₃ comprise, each independently, H, an alkyl group, an alkenyl group, an alkynyl group, an aromatic group, or a heterocyclic group; and

n is a distribution of integers between 1 and 100,000 with an average value of greater than one; and

(b) a charge generating compound.

The asterisks (*) indicate terminal groups of the polymer of Formula (I), which may vary between different polymer units depending on the state of the particular polymerization process at the end of the polymerization step.

The organophotoreceptor may be provided, for example, in the form of a plate, a flexible belt, a flexible disk, a sheet, a rigid drum, or a sheet around a rigid or compliant drum. In one embodiment, the organophotoreceptor includes: (a) a photoconductive element comprising the charge transport material, the charge generating compound, a second charge transport material, and a polymeric binder; and (b) the electrically conductive substrate.

In a second aspect, the invention features an electrophotographic imaging apparatus that comprises (a) a light imaging component; and (b) the above-described organophotoreceptor oriented to receive light from the light imaging component. The apparatus can further comprise a toner dispenser, such as a liquid toner dispenser. The method of electrophotographic imaging with photoreceptors containing the above noted charge transport materials is also described.

In a third aspect, the invention features an electrophotographic imaging process that includes (a) applying an electrical charge to a surface of the above-described organophotoreceptor; (b) imagewise exposing the surface of the organophotoreceptor to radiation to dissipate charge in selected areas and thereby form a pattern of at least relatively charged and uncharged areas on the surface; (c) contacting the surface with a toner, such as a liquid toner that includes a dispersion of colorant particles in an organic liquid, to create a toned image; and (d) transferring the toned image to a substrate.

In a fourth aspect, the invention features a charge transport material comprising a polymer having Formula (I) above.

In a fifth aspect, the invention features a method for forming a polymeric charge transport material, the method comprising the step of co-polymerizing a bridging compound having a bridging group and at least two functional groups with a charge transport material having the formula:

where X₃ and X₄ are, each independently, a linking group, such as a —(CH₂)_(p)— group, where p is an integer between 1 and 10, inclusive, and one or more of the methylene groups is optionally replaced by O, S, N, C, B, Si, P, C═O, O═S═O, an NR_(g) group, a CR_(h) group, a CR_(i)R_(j) group, or a SiR_(k)R_(l) where R_(g), R_(h), R_(i), R_(j), R_(k), and R_(l) are, each independently, a bond, H, a hydroxyl group, a thiol group, a carboxyl group, an amino group, an alkyl group, an alkoxy group, an alkenyl group, an alkynyl group, a heterocyclic group, an aromatic group, or a part of a ring group, such as cycloalkyl groups or a benzo group;

Ar comprises an aromatic group, such as an aromatic C₆H₃ group;

R₁, R₂, and R₃ comprise, each independently, H, an alkyl group, an alkenyl group, an alkynyl group, an aromatic group, or a heterocyclic group; and

E₁ and E₂ are, each independently, a reactive ring group, such as an epoxy group, a thiiranyl group, an aziridino group, and an oxetanyl group.

In some embodiments, the bridging group is a —(CH₂)_(k)— group, where k is an integer between 1 and 30, inclusive, and one or more of the methylene groups is optionally replaced by O, S, N, C, B, Si, P, C═O, O═S═O, an NR_(m) group, a CR_(n) group, a CR_(o)R_(p) group, or a SiR_(q)R_(r) where R_(m), R_(n), R_(o), R_(p), R_(q), and R_(r) are, each independently, a bond, H, a hydroxyl group, a thiol group, a carboxyl group, an amino group, an alkyl group, an alkoxy group, an alkenyl group, an alkynyl group, a heterocyclic group, an aromatic group, or a part of a ring group, such as cycloalkyl groups or a benzo group. In other embodiments, the functional groups, each independently, are a hydroxyl group, a thiol group, a carboxyl group, and an amino group.

The invention provides suitable charge transport materials for organophotoreceptors featuring a combination of good chemical and electrostatic properties. These photoreceptors can be used successfully with toners, including liquid toners, to produce high quality images. The high quality of the imaging system can be maintained after repeated cycling.

Other features and advantages of the invention will be apparent from the following description of the particular embodiments thereof, and from the claims.

DETAILED DESCRIPTION OF THE INVENTION

An organophotoreceptor as described herein has an electrically conductive substrate and a photoconductive element including a charge generating compound and a charge transport material comprising a polymer having repeating aromatic azine groups. These charge transport materials have desirable properties as evidenced by their performance in organophotoreceptors for electrophotography. In particular, the charge transport materials of this invention have high charge carrier mobilities and good compatibility with various binder materials, and possess excellent electrophotographic properties. The organophotoreceptors according to this invention generally have a high photosensitivity, a low residual potential, and a high stability with respect to cycle testing, crystallization, and organophotoreceptor bending and stretching. The organophotoreceptors are particularly useful in laser printers and the like as well as fax machines, photocopiers, scanners and other electronic devices based on electrophotography. The use of these charge transport materials is described in more detail below in the context of laser printer use, although their application in other devices operating by electrophotography can be generalized from the discussion below.

To produce high quality images, particularly after multiple cycles, it is desirable for the charge transport materials to form a homogeneous mixture with the polymeric binder and remain approximately homogeneously distributed through the organophotoreceptor material during the cycling of the material. In addition, it is desirable to increase the amount of charge that the charge transport material can accept (indicated by a parameter known as the acceptance voltage or “V_(acc)”), and to reduce retention of that charge upon discharge (indicated by a parameter known as the discharge voltage or “V_(dis)”).

Charge transport materials may comprise monomeric molecules (e.g., N-ethyl-carbazolo-3-aldehyde-N-methyl-N-phenyl-hydrazone, dimeric molecules (e.g., disclosed in U.S. Pat. Nos. 6,140,004 and 6,670,085), or polymeric compositions (e.g., poly(vinylcarbazole)). Furthermore, the charge transport materials can be classified as a charge transport compound or an electron transport compound. There are many charge transport compounds and electron transport compounds known in the art for electrophotography. Non-limiting examples of charge transport compounds include, for example, pyrazoline derivatives, fluorene derivatives, oxadiazole derivatives, stilbene derivatives, enamine derivatives, enamine stilbene derivatives, hydrazone derivatives, carbazole hydrazone derivatives, (N,N-disubstituted)arylamines such as triaryl amines, polyvinyl carbazole, polyvinyl pyrene, polyacenaphthylene, and the charge transport compounds described in U.S. Pat. Nos. 6,689,523, 6,670,085, and 6,696,209, and U.S. patent application Ser. Nos. 10/431,135, 10/431,138, 10/699,364, 10/663,278, 10/699,581, 10/449,554, 10/748,496, 10/789,094, 10/644,547, 10/749,174, 10/749,171, 10/749,418, 10/699,039, 10/695,581, 10/692,389, 10/634,164, 10/663,970, 10/749,164, 10/772,068, 10/749,178, 10/758,869, 10/695,044, 10/772,069, 10/789,184, 10/789,077, 10/775,429, 10/775,429, 10/670,483, 10/671,255, 10/663,971, 10/760,039. All the above patents and patent applications are incorporated herein by reference.

Non-limiting examples of electron transport compounds include, for example, bromoaniline, tetracyanoethylene, tetracyanoquinodimethane, 2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitro-9-fluorenone, 2,4,5,7-tetranitroxanthone, 2,4,8-trinitrothioxanthone, 2,6,8-trinitro-indeno[1,2-b]thiophene-4-one, and 1,3,7-trinitrodibenzo thiophene-5,5-dioxide, (2,3-diphenyl-1-indenylidene)malononitrile, 4H-thiopyran-1,1-dioxide and its derivatives such as 4-dicyanomethylene-2,6-diphenyl-4H-thiopyran-1,1-dioxide, 4-dicyanomethylene-2,6-di-m-tolyl-4H-thiopyran-1,1-dioxide, and unsymmetrically substituted 2,6-diaryl-4H-thiopyran-1,1-dioxide such as 4H-1,1-dioxo-2-(p-isopropylphenyl)-6-phenyl-4-(dicyanomethylidene)thiopyran and 4H-1,1-dioxo-2-(p-isopropylphenyl)-6-(2-thienyl)-4-(dicyanomethylidene)thiopyran, derivatives of phospha-2,5-cyclohexadiene, alkoxycarbonyl-9-fluorenylidene)malononitrile derivatives such as (4-n-butoxycarbonyl-9-fluorenylidene)malononitrile, (4-phenethoxycarbonyl-9-fluorenylidene)malononitrile, (4-carbitoxy-9-fluorenylidene)malononitrile, and diethyl(4-n-butoxycarbonyl-2,7-dinitro-9-fluorenylidene)malonate, anthraquinodimethane derivatives such as 11,11,12,12-tetracyano-2-alkylanthraquinodimethane and 11,11-dicyano-12,12-bis(ethoxycarbonyl)anthraquinodimethane, anthrone derivatives such as 1-chloro-10-[bis(ethoxycarbonyl)methylene]anthrone, 1,8-dichloro-10-[bis(ethoxycarbonyl) methylene]anthrone, 1,8-dihydroxy-10-[bis(ethoxycarbonyl)methylene] anthrone, and 1-cyano-10-[bis(ethoxycarbonyl)methylene)anthrone, 7-nitro-2-aza-9-fluroenylidene-malononitrile, diphenoquinone derivatives, benzoquinone derivatives, naphtoquinone derivatives, quinine derivatives, tetracyanoethylenecyanoethylene, 2,4,8-trinitro thioxantone, dinitrobenzene derivatives, dinitroanthracene derivatives, dinitroacridine derivatives, nitroanthraquinone derivatives, dinitroanthraquinone derivatives, succinic anhydride, maleic anhydride, dibromo maleic anhydride, pyrene derivatives, carbazole derivatives, hydrazone derivatives, N,N-dialkylaniline derivatives, diphenylamine derivatives, triphenylamine derivatives, triphenylmethane derivatives, tetracyano quinodimethane, 2,4,5,7-tetranitro-9-fluorenone, 2,4,7-trinitro-9-dicyanomethylene fluorenone, 2,4,5,7-tetranitroxanthone derivatives, 2,4,8-trinitrothioxanthone derivatives, 1,4,5,8-naphthalene bis-dicarboximide derivatives as described in U.S. Pat. Nos. 5,232,800, 4,468,444, and 4,442,193 and phenylazoquinolide derivatives as described in U.S. Pat. No. 6,472,514. In some embodiments of interest, the electron transport compound comprises an (alkoxycarbonyl-9-fluorenylidene)malononitrile derivative, such as (4-n-butoxycarbonyl-9-fluorenylidene)malononitrile, and 1,4,5,8-naphthalene bis-dicarboximide derivatives.

Although there are many charge transport materials available, there is a need for other charge transport materials to meet the various requirements of particular electrophotography applications.

In electrophotography applications, a charge-generating compound within an organophotoreceptor absorbs light to form electron-hole pairs. These electrons and holes can be transported over an appropriate time frame under a large electric field to discharge locally a surface charge that is generating the field. The discharge of the field at a particular location results in a surface charge pattern that essentially matches the pattern drawn with the light. This charge pattern then can be used to guide toner deposition. The charge transport materials described herein are especially effective at transporting charge, and in particular holes from the electron-hole pairs formed by the charge generating compound. In some embodiments, a specific electron transport compound or charge transport compound can also be used along with the charge transport material of this invention.

The layer or layers of materials containing the charge generating compound and the charge transport materials are within an organophotoreceptor. To print a two dimensional image using the organophotoreceptor, the organophotoreceptor has a two dimensional surface for forming at least a portion of the image. The imaging process then continues by cycling the organophotoreceptor to complete the formation of the entire image and/or for the processing of subsequent images.

The organophotoreceptor may be provided in the form of a plate, a flexible belt, a disk, a rigid drum, a sheet around a rigid or compliant drum, or the like. The charge transport material can be in the same layer as the charge generating compound and/or in a different layer from the charge generating compound. Additional layers can be used also, as described further below.

In some embodiments, the organophotoreceptor material comprises, for example: (a) a charge transport layer comprising the charge transport material and a polymeric binder; (b) a charge generating layer comprising the charge generating compound and a polymeric binder; and (c) the electrically conductive substrate. The charge transport layer may be intermediate between the charge generating layer and the electrically conductive substrate. Alternatively, the charge generating layer may be intermediate between the charge transport layer and the electrically conductive substrate. In further embodiments, the organophotoreceptor material has a single layer with both a charge transport material and a charge generating compound within a polymeric binder.

The organophotoreceptors can be incorporated into an electrophotographic imaging apparatus, such as laser printers. In these devices, an image is formed from physical embodiments and converted to a light image that is scanned onto the organophotoreceptor to form a surface latent image. The surface latent image can be used to attract toner onto the surface of the organophotoreceptor, in which the toner image is the same or the negative of the light image projected onto the organophotoreceptor. The toner can be a liquid toner or a dry toner. The toner is subsequently transferred, from the surface of the organophotoreceptor, to a receiving surface, such as a sheet of paper. After the transfer of the toner, the surface is discharged, and the material is ready to cycle again. The imaging apparatus can further comprise, for example, a plurality of support rollers for transporting a paper receiving medium and/or for movement of the photoreceptor, a light imaging component with suitable optics to form the light image, a light source, such as a laser, a toner source and delivery system and an appropriate control system.

An electrophotographic imaging process generally can comprise (a) applying an electrical charge to a surface of the above-described organophotoreceptor; (b) imagewise exposing the surface of the organophotoreceptor to radiation to dissipate charge in selected areas and thereby form a pattern of charged and uncharged areas on the surface; (c) exposing the surface with a toner, such as a liquid toner that includes a dispersion of colorant particles in an organic liquid to create a toner image, to attract toner to the charged or discharged regions of the organophotoreceptor; and (d) transferring the toner image to a substrate.

As described herein, an organophotoreceptor comprises a charge transport material having the formula:

where X₁ and X₂ are, each independently, a linking group, such as —(CH₂)_(m)— group, where m is an integer between 1 and 20, inclusive, and one or more of the methylene groups is optionally replaced by O, S, N, C, B, Si, P, C═O, O═S═O, an NR_(a) group, a CR_(b) group, a CR_(c)R_(d) group, or a SiR_(e)R_(f) where R_(a), R_(b), R, R_(d), R_(e), and R_(f) are, each independently, a bond, H, a hydroxyl group, a thiol group, a carboxyl group, an amino group, an alkyl group, an alkoxy group, an alkenyl group, an alkynyl group, a heterocyclic group, an aromatic group, or a part of a ring group, such as cycloalkyl groups or a benzo group;

Ar comprises an aromatic group;

R₁, R₂, and R₃ comprise, each independently, H, an alkyl group, an alkenyl group, an alkynyl group, an aromatic group, or a heterocyclic group; and

n is a distribution of integers between 1 and 100,000 with an average value of greater than one.

A heterocyclic group includes any monocyclic or polycyclic (e.g., bicyclic, tricyclic, etc.) ring compound having at least a heteroatom (e.g., O, S, N, P, B, Si, etc.) in the ring.

An aromatic group can be any conjugated ring system containing 4n+2 pi-electrons. There are many criteria available for determining aromaticity. A widely employed criterion for the quantitative assessment of aromaticity is the resonance energy. Specifically, an aromatic group has a resonance energy. In some embodiments, the resonance energy of the aromatic group is at least 10 KJ/mol. In further embodiments, the resonance energy of the aromatic group is greater than 0.1 KJ/mol. Aromatic groups may be classified as an aromatic heterocyclic group which contains at least a heteroatom in the 4n+2 pi-electron ring, or as an aryl group which does not contain a heteroatom in the 4n+2 pi-electron ring. The aromatic group may comprise a combination of aromatic heterocyclic group and aryl group. Nonetheless, either the aromatic heterocyclic or the aryl group may have at least one heteroatom in a substituent attached to the 4n+2 pi-electron ring. Furthermore, either the aromatic heterocyclic or the aryl group may comprise a monocyclic or polycyclic (such as bicyclic, tricyclic, etc.) ring.

Non-limiting examples of the aromatic heterocyclic group are furanyl, thiophenyl, pyrrolyl, indolyl, carbazolyl, benzofuranyl, benzothiophenyl, dibenzofuranyl, dibenzothiophenyl, pyridinyl, pyridazinyl, pyrimidinyl, pyrazinyl, triazinyl, tetrazinyl, petazinyl, quinolinyl, isoquinolinyl, cinnolinyl, phthalazinyl, quinazolinyl, quinoxalinyl, naphthyridinyl, acridinyl, phenanthridinyl, phenanthrolinyl, anthyridinyl, purinyl, pteridinyl, alloxazinyl, phenazinyl, phenothiazinyl, phenoxazinyl, phenoxathiinyl, dibenzo(1,4)dioxinyl, thianthrenyl, and a combination thereof. The aromatic heterocyclic group may also include any combination of the above aromatic heterocyclic groups bonded together either by a bond (as in bicarbazolyl) or by a linking group (as in 1,6 di(10H-10-phenothiazinyl)hexane). The linking group may include an aliphatic group, an aromatic group, a heterocyclic group, or a combination thereof. Furthermore, either an aliphatic group or an aromatic group within a linking group may comprise at least one heteroatom such as O, S, Si, and N.

Non-limiting examples of the aryl group are phenyl, naphthyl, benzyl, or tolanyl group, sexiphenylene, phenanthrenyl, anthracenyl, coronenyl, and tolanylphenyl. The aryl group may also include any combination of the above aryl groups bonded together either by a bond (as in biphenyl group) or a linking group (as in stilbenyl, diphenyl sulfone, an arylamine group). The linking group may include an aliphatic group, an aromatic group, a heterocyclic group, or a combination thereof. Furthermore, either an aliphatic group or an aromatic group within a linking group may comprise at least one heteroatom such as O, S, Si, and N.

Substitution is liberally allowed on the chemical groups to affect various physical effects on the properties of the compounds, such as mobility, sensitivity, solubility, stability, and the like, as is known generally in the art. In the description of chemical substituents, there are certain practices common to the art that are reflected in the use of language. The term group indicates that the generically recited chemical entity (e.g., alkyl group, phenyl group, aromatic group, etc.) may have any substituent thereon which is consistent with the bond structure of that group. For example, where the term ‘alkyl group’ is used, that term would not only include unsubstituted linear, branched and cyclic alkyls, such as methyl, ethyl, isopropyl, tert-butyl, cyclohexyl, dodecyl and the like, but also substituents having heteroatom, such as 3-ethoxylpropyl, 4-(N,N-diethylamino)butyl, 3-hydroxypentyl, 2-thiolhexyl, 1,2,3-tribromoopropyl, and the like, and aromatic group, such as phenyl, naphthyl, carbazolyl, pyrrole, and the like. However, as is consistent with such nomenclature, no substitution would be included within the term that would alter the fundamental bond structure of the underlying group. For example, where a phenyl group is recited, substitution such as 2- or 4-aminophenyl, 2- or 4-(N,N-disubstituted)aminophenyl, 2,4-dihydroxyphenyl, 2,4,6-trithiophenyl, 2,4,6-trimethoxyphenyl and the like would be acceptable within the terminology, while substitution of 1,1,2,2,3,3-hexamethylphenyl would not be acceptable as that substitution would require the ring bond structure of the phenyl group to be altered to a non-aromatic form. Where the term moiety is used, such as alkyl moiety or phenyl moiety, that terminology indicates that the chemical material is not substituted. Where the term alkyl moiety is used, that term represents only an unsubstituted alkyl hydrocarbon group, whether branched, straight chain, or cyclic.

Organophotoreceptors

The organophotoreceptor may be, for example, in the form of a plate, a sheet, a flexible belt, a disk, a rigid drum, or a sheet around a rigid or compliant drum, with flexible belts and rigid drums generally being used in commercial embodiments. The organophotoreceptor may comprise, for example, an electrically conductive substrate and on the electrically conductive substrate a photoconductive element in the form of one or more layers. The photoconductive element can comprise both a charge transport material and a charge generating compound in a polymeric binder, which may or may not be in the same layer, as well as a second charge transport material such as a charge transport compound or an electron transport compound in some embodiments. For example, the charge transport material and the charge generating compound can be in a single layer. In other embodiments, however, the photoconductive element comprises a bilayer construction featuring a charge generating layer and a separate charge transport layer. The charge generating layer may be located intermediate between the electrically conductive substrate and the charge transport layer. Alternatively, the photoconductive element may have a structure in which the charge transport layer is intermediate between the electrically conductive substrate and the charge generating layer.

The electrically conductive substrate may be flexible, for example in the form of a flexible web or a belt, or inflexible, for example in the form of a drum. A drum can have a hollow cylindrical structure that provides for attachment of the drum to a drive that rotates the drum during the imaging process. Typically, a flexible electrically conductive substrate comprises an electrically insulating substrate and a thin layer of electrically conductive material onto which the photoconductive material is applied.

The electrically insulating substrate may be paper or a film forming polymer such as polyester (e.g., polyethylene terephthalate or polyethylene naphthalate), polyimide, polysulfone, polypropylene, nylon, polyester, polycarbonate, polyvinyl resin, polyvinyl fluoride, polystyrene and the like. Specific examples of polymers for supporting substrates included, for example, polyethersulfone (STABAR™ S-100, available from ICI), polyvinyl fluoride (Tedlar®, available from E.I. DuPont de Nemours & Company), polybisphenol-A polycarbonate (MAKROFOL™, available from Mobay Chemical Company) and amorphous polyethylene terephthalate (MELINAR™, available from ICI Americas, Inc.). The electrically conductive materials may be graphite, dispersed carbon black, iodine, conductive polymers such as polypyrroles and Calgon® conductive polymer 261 (commercially available from Calgon Corporation, Inc., Pittsburgh, Pa.), metals such as aluminum, titanium, chromium, brass, gold, copper, palladium, nickel, or stainless steel, or metal oxide such as tin oxide or indium oxide. In embodiments of particular interest, the electrically conductive material is aluminum. Generally, the photoconductor substrate has a thickness adequate to provide the required mechanical stability. For example, flexible web substrates generally have a thickness from about 0.01 to about 1 mm, while drum substrates generally have a thickness from about 0.5 mm to about 2 mm.

The charge generating compound is a material that is capable of absorbing light to generate charge carriers (such as a dye or pigment). Non-limiting examples of suitable charge generating compounds include, for example, metal-free phthalocyanines (e.g., ELA 8034 metal-free phthalocyanine available from H.W. Sands, Inc. or Sanyo Color Works, Ltd., CGM-X01), metal phthalocyanines such as titanium phthalocyanine, copper phthalocyanine, oxytitanium phthalocyanine (also referred to as titanyl oxyphthalocyanine, and including any crystalline phase or mixtures of crystalline phases that can act as a charge generating compound), hydroxygallium phthalocyanine, squarylium dyes and pigments, hydroxy-substituted squarylium pigments, perylimides, polynuclear quinones available from Allied Chemical Corporation under the trade name INDOFAST™ Double Scarlet, INDOFAST™ Violet Lake B, INDOFAST™ Brilliant Scarlet and INDOFAST™ Orange, quinacridones available from DuPont under the trade name MONASTRAL™ Red, MONASTRAL™ Violet and MONASTRAL™ Red Y, naphthalene 1,4,5,8-tetracarboxylic acid derived pigments including the perinones, tetrabenzoporphyrins and tetranaphthaloporphyrins, indigo- and thioindigo dyes, benzothioxanthene-derivatives, perylene 3,4,9,10-tetracarboxylic acid derived pigments, polyazo-pigments including bisazo-, trisazo- and tetrakisazo-pigments, polymethine dyes, dyes containing quinazoline groups, tertiary amines, amorphous selenium, selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic and selenium-arsenic, cadmium sulphoselenide, cadmium selenide, cadmium sulphide, and mixtures thereof. For some embodiments, the charge generating compound comprises oxytitanium phthalocyanine (e.g., any phase thereof), hydroxygallium phthalocyanine or a combination thereof.

The photoconductive layer of this invention may optionally contain a second charge transport material which may be a charge transport compound, an electron transport compound, or a combination of both. Generally, any charge transport compound or electron transport compound known in the art can be used as the second charge transport material.

An electron transport compound and a UV light stabilizer can have a synergistic relationship for providing desired electron flow within the photoconductor. The presence of the UV light stabilizers alters the electron transport properties of the electron transport compounds to improve the electron transporting properties of the composite. UV light stabilizers can be ultraviolet light absorbers or ultraviolet light inhibitors that trap free radicals.

UV light absorbers can absorb ultraviolet radiation and dissipate it as heat. UV light inhibitors are thought to trap free radicals generated by the ultraviolet light and after trapping of the free radicals, subsequently to regenerate active stabilizer moieties with energy dissipation. In view of the synergistic relationship of the UV stabilizers with electron transport compounds, the particular advantages of the UV stabilizers may not be their UV stabilizing abilities, although the UV stabilizing ability may be further advantageous in reducing degradation of the organophotoreceptor over time. The improved synergistic performance of organophotoreceptors with layers comprising both an electron transport compound and a UV stabilizer are described further in copending U.S. patent application Ser. No. 10/425,333 filed on Apr. 28, 2003 to Zhu, entitled “Organophotoreceptor With A Light Stabilizer,” incorporated herein by reference.

Non-limiting examples of suitable light stabilizer include, for example, hindered trialkylamines such as Tinuvin 144 and Tinuvin 292 (from Ciba Specialty Chemicals, Terrytown, N.Y.), hindered alkoxydialkylamines such as Tinuvin 123 (from Ciba Specialty Chemicals), benzotriazoles such as Tinuvan 328, Tinuvin 900 and Tinuvin 928 (from Ciba Specialty Chemicals), benzophenones such as Sanduvor 3041 (from Clariant Corp., Charlotte, N.C.), nickel compounds such as Arbestab (from Robinson Brothers Ltd, West Midlands, Great Britain), salicylates, cyanocinnamates, benzylidene malonates, benzoates, oxanilides such as Sanduvor VSU (from Clariant Corp., Charlotte, N.C.), triazines such as Cyagard UV-1164 (from Cytec Industries Inc., N.J.), polymeric sterically hindered amines such as Luchem (from Atochem North America, Buffalo, N.Y.). In some embodiments, the light stabilizer is selected from the group consisting of hindered trialkylamines having the following formula:

where R₁, R₂, R₃, R₄, R₆, R₇, R₈, R₁₀, R₁₁, R₁₂, R₁₃, R₁₄, R₁₅ are, each independently, hydrogen, alkyl group, or ester, or ether group; and R₅, R₉, and R₁₄ are, each independently, alkyl group; and X is a linking group selected from the group consisting of —O—CO—(CH₂)_(m)—CO—O— where m is between 2 to 20.

The binder generally is capable of dispersing or dissolving the charge transport material (in the case of the charge transport layer or a single layer construction), the charge generating compound (in the case of the charge generating layer or a single layer construction) and/or an electron transport compound for appropriate embodiments. Examples of suitable binders for both the charge generating layer and charge transport layer generally include, for example, polystyrene-co-butadiene, polystyrene-co-acrylonitrile, modified acrylic polymers, polyvinyl acetate, styrene-alkyd resins, soya-alkyl resins, polyvinylchloride, polyvinylidene chloride, polyacrylonitrile, polycarbonates, polyacrylic acid, polyacrylates, polymethacrylates, styrene polymers, polyvinyl butyral, alkyd resins, polyamides, polyurethanes, polyesters, polysulfones, polyethers, polyketones, phenoxy resins, epoxy resins, silicone resins, polysiloxanes, poly(hydroxyether) resins, polyhydroxystyrene resins, novolak, poly(phenylglycidyl ether)-co-dicyclopentadiene, copolymers of monomers used in the above-mentioned polymers, and combinations thereof. Specific suitable binders include, for example, polyvinyl butyral, polycarbonate, and polyester. Non-limiting examples of polyvinyl butyral include BX-1 and BX-5 from Sekisui Chemical Co. Ltd., Japan. Non-limiting examples of suitable polycarbonate include polycarbonate A which is derived from bisphenol-A (e.g. Iupilon-A from Mitsubishi Engineering Plastics, or Lexan 145 from General Electric); polycarbonate Z which is derived from cyclohexylidene bisphenol (e.g. Iupilon-Z from Mitsubishi Engineering Plastics Corp, White Plain, N.Y.); and polycarbonate C which is derived from methylbisphenol A (from Mitsubishi Chemical Corporation). Non-limiting examples of suitable polyester binders include ortho-polyethylene terephthalate (e.g. OPET TR-4 from Kanebo Ltd., Yamaguchi, Japan).

Suitable optional additives for any one or more of the layers include, for example, antioxidants, coupling agents, dispersing agents, curing agents, surfactants, and combinations thereof.

The photoconductive element overall typically has a thickness from about 10 microns to about 45 microns. In the dual layer embodiments having a separate charge generating layer and a separate charge transport layer, charge generation layer generally has a thickness form about 0.5 microns to about 2 microns, and the charge transport layer has a thickness from about 5 microns to about 35 microns. In embodiments in which the charge transport material and the charge generating compound are in the same layer, the layer with the charge generating compound and the charge transport composition generally has a thickness from about 7 microns to about 30 microns. In embodiments with a distinct electron transport layer, the electron transport layer has an average thickness from about 0.5 microns to about 10 microns and in further embodiments from about 1 micron to about 3 microns. In general, an electron transport overcoat layer can increase mechanical abrasion resistance, increases resistance to carrier liquid and atmospheric moisture, and decreases degradation of the photoreceptor by corona gases. A person of ordinary skill in the art will recognize that additional ranges of thickness within the explicit ranges above are contemplated and are within the present disclosure.

Generally, for the organophotoreceptors described herein, the charge generation compound is in an amount from about 0.5 to about 25 weight percent, in further embodiments in an amount from about 1 to about 15 weight percent, and in other embodiments in an amount from about 2 to about 10 weight percent, based on the weight of the photoconductive layer. The charge transport material is in an amount from about 10 to about 80 weight percent, based on the weight of the photoconductive layer, in further embodiments in an amount from about 35 to about 60 weight percent, and in other embodiments from about 45 to about 55 weight percent, based on the weight of the photoconductive layer. The optional second charge transport material, when present, can be in an amount of at least about 2 weight percent, in other embodiments from about 2.5 to about 25 weight percent, based on the weight of the photoconductive layer, and in further embodiments in an amount from about 4 to about 20 weight percent, based on the weight of the photoconductive layer. The binder is in an amount from about 15 to about 80 weight percent, based on the weight of the photoconductive layer, and in further embodiments in an amount from about 20 to about 75 weight percent, based on the weight of the photoconductive layer. A person of ordinary skill in the art will recognize that additional ranges within the explicit ranges of compositions are contemplated and are within the present disclosure.

For the dual layer embodiments with a separate charge generating layer and a charge transport layer, the charge generation layer generally comprises a binder in an amount from about 10 to about 90 weight percent, in further embodiments from about 15 to about 80 weight percent and in some embodiments in an amount from about 20 to about 75 weight percent, based on the weight of the charge generation layer. The optional charge transport material in the charge generating layer, if present, generally can be in an amount of at least about 2.5 weight percent, in further embodiments from about 4 to about 30 weight percent and in other embodiments in an amount from about 10 to about 25 weight percent, based on the weight of the charge generating layer. The charge transport layer generally comprises a binder in an amount from about 20 weight percent to about 70 weight percent and in further embodiments in an amount from about 30 weight percent to about 50 weight percent. A person of ordinary skill in the art will recognize that additional ranges of binder concentrations for the dual layer embodiments within the explicit ranges above are contemplated and are within the present disclosure.

For the embodiments with a single layer having a charge generating compound and a charge transport material, the photoconductive layer generally comprises a binder, a charge transport material, and a charge generation compound. The charge generation compound can be in an amount from about 0.05 to about 25 weight percent and in further embodiment in an amount from about 2 to about 15 weight percent, based on the weight of the photoconductive layer. The charge transport material can be in an amount from about 10 to about 80 weight percent, in other embodiments from about 25 to about 65 weight percent, in additional embodiments from about 30 to about 60 weight percent and in further embodiments in an amount from about 35 to about 55 weight percent, based on the weight of the photoconductive layer, with the remainder of the photoconductive layer comprising the binder, and optionally additives, such as any conventional additives. A single layer with a charge transport composition and a charge generating compound generally comprises a binder in an amount from about 10 weight percent to about 75 weight percent, in other embodiments from about 20 weight percent to about 60 weight percent, and in further embodiments from about 25 weight percent to about 50 weight percent. Optionally, the layer with the charge generating compound and the charge transport material may comprise a second charge transport material. The optional second charge transport material, if present, generally can be in an amount of at least about 2.5 weight percent, in further embodiments from about 4 to about 30 weight percent and in other embodiments in an amount from about 10 to about 25 weight percent, based on the weight of the photoconductive layer. A person of ordinary skill in the art will recognize that additional composition ranges within the explicit compositions ranges for the layers above are contemplated and are within the present disclosure.

In general, any layer with an electron transport layer can advantageously further include a UV light stabilizer. In particular, the electron transport layer generally can comprise an electron transport compound, a binder, and an optional UV light stabilizer. An overcoat layer comprising an electron transport compound is described further in copending U.S. patent application Ser. No. 10/396,536 to Zhu et al. entitled, “Organophotoreceptor With An Electron Transport Layer,” incorporated herein by reference. For example, an electron transport compound as described above may be used in the release layer of the photoconductors described herein. The electron transport compound in an electron transport layer can be in an amount from about 10 to about 50 weight percent, and in other embodiments in an amount from about 20 to about 40 weight percent, based on the weight of the electron transport layer. A person of ordinary skill in the art will recognize that additional ranges of compositions within the explicit ranges are contemplated and are within the present disclosure.

The UV light stabilizer, if present, in any one or more appropriate layers of the photoconductor generally is in an amount from about 0.5 to about 25 weight percent and in some embodiments in an amount from about 1 to about 10 weight percent, based on the weight of the particular layer. A person of ordinary skill in the art will recognize that additional ranges of compositions within the explicit ranges are contemplated and are within the present disclosure.

For example, the photoconductive layer may be formed by dispersing or dissolving the components, such as one or more of a charge generating compound, the charge transport material of this invention, a second charge transport material such as a charge transport compound or an electron transport compound, a UV light stabilizer, and a polymeric binder in organic solvent, coating the dispersion and/or solution on the respective underlying layer and drying the coating. In particular, the components can be dispersed by high shear homogenization, ball-milling, attritor milling, high energy bead (sand) milling or other size reduction processes or mixing means known in the art for effecting particle size reduction in forming a dispersion.

The photoreceptor may optionally have one or more additional layers as well. An additional layer can be, for example, a sub-layer or an overcoat layer, such as a barrier layer, a release layer, a protective layer, or an adhesive layer. A release layer or a protective layer may form the uppermost layer of the photoconductor element. A barrier layer may be sandwiched between the release layer and the photoconductive element or used to overcoat the photoconductive element. The barrier layer provides protection from abrasion to the underlayers. An adhesive layer locates and improves the adhesion between a photoconductive element, a barrier layer and a release layer, or any combination thereof. A sub-layer is a charge blocking layer and locates between the electrically conductive substrate and the photoconductive element. The sub-layer may also improve the adhesion between the electrically conductive substrate and the photoconductive element.

Suitable barrier layers include, for example, coatings such as crosslinkable siloxanol-colloidal silica coating and hydroxylated silsesquioxane-colloidal silica coating, and organic binders such as polyvinyl alcohol, methyl vinyl ether/maleic anhydride copolymer, casein, polyvinyl pyrrolidone, polyacrylic acid, gelatin, starch, polyurethanes, polyimides, polyesters, polyamides, polyvinyl acetate, polyvinyl chloride, polyvinylidene chloride, polycarbonates, polyvinyl butyral, polyvinyl acetoacetal, polyvinyl formal, polyacrylonitrile, polymethyl methacrylate, polyacrylates, polyvinyl carbazoles, copolymers of monomers used in the above-mentioned polymers, vinyl chloride/vinyl acetate/vinyl alcohol terpolymers, vinyl chloride/vinyl acetate/maleic acid terpolymers, ethylene/vinyl acetate copolymers, vinyl chloride/vinylidene chloride copolymers, cellulose polymers, and mixtures thereof. The above barrier layer polymers optionally may contain small inorganic particles such as fumed silica, silica, titania, alumina, zirconia, or a combination thereof. Barrier layers are described further in U.S. Pat. No. 6,001,522 to Woo et al., entitled “Barrier Layer For Photoconductor Elements Comprising An Organic Polymer And Silica,” incorporated herein by reference. The release layer topcoat may comprise any release layer composition known in the art. In some embodiments, the release layer is a fluorinated polymer, siloxane polymer, fluorosilicone polymer, silane, polyethylene, polypropylene, polyacrylate, or a combination thereof. The release layers can comprise crosslinked polymers.

The release layer may comprise, for example, any release layer composition known in the art. In some embodiments, the release layer comprises a fluorinated polymer, siloxane polymer, fluorosilicone polymer, polysilane, polyethylene, polypropylene, polyacrylate, poly(methyl methacrylate-co-methacrylic acid), urethane resins, urethane-epoxy resins, acrylated-urethane resins, urethane-acrylic resins, or a combination thereof. In further embodiments, the release layers comprise crosslinked polymers.

The protective layer can protect the organophotoreceptor from chemical and mechanical degradation. The protective layer may comprise any protective layer composition known in the art. In some embodiments, the protective layer is a fluorinated polymer, siloxane polymer, fluorosilicone polymer, polysilane, polyethylene, polypropylene, polyacrylate, poly(methyl methacrylate-co-methacrylic acid), urethane resins, urethane-epoxy resins, acrylated-urethane resins, urethane-acrylic resins, or a combination thereof. In some embodiments of particular interest, the release layers are crosslinked polymers.

An overcoat layer may comprise an electron transport compound as described further in copending U.S. patent application Ser. No. 10/396,536, filed on Mar. 25, 2003 to Zhu et al. entitled, “Organoreceptor With An Electron Transport Layer,” incorporated herein by reference. For example, an electron transport compound, as described above, may be used in the release layer of this invention. The electron transport compound in the overcoat layer can be in an amount from about 2 to about 50 weight percent, and in other embodiments in an amount from about 10 to about 40 weight percent, based on the weight of the release layer. A person of ordinary skill in the art will recognize that additional ranges of composition within the explicit ranges are contemplated and are within the present disclosure.

Generally, adhesive layers comprise a film forming polymer, such as polyester, polyvinylbutyral, polyvinylpyrrolidone, polyurethane, polymethyl methacrylate, poly(hydroxy amino ether) and the like. Barrier and adhesive layers are described further in U.S. Pat. No. 6,180,305 to Ackley et al., entitled “Organic Photoreceptors for Liquid Electrophotography,” incorporated herein by reference.

Sub-layers can comprise, for example, polyvinylbutyral, organosilanes, hydrolyzable silanes, epoxy resins, polyesters, polyamides, polyurethanes, cellulosics, and the like. In some embodiments, the sub-layer has a dry thickness between about 20 Angstroms and about 20,000 Angstroms. Sublayers containing metal oxide conductive particles can be between about 1 and about 25 microns thick. A person of ordinary skill in the art will recognize that additional ranges of compositions and thickness within the explicit ranges are contemplated and are within the present disclosure.

The charge transport materials as described herein, and photoreceptors including these compounds, are suitable for use in an imaging process with either dry or liquid toner development. For example, any dry toners and liquid toners known in the art may be used in the process and the apparatus of this invention. Liquid toner development can be desirable because it offers the advantages of providing higher resolution images and requiring lower energy for image fixing compared to dry toners. Examples of suitable liquid toners are known in the art. Liquid toners generally comprise toner particles dispersed in a carrier liquid. The toner particles can comprise a colorant/pigment, a resin binder, and/or a charge director. In some embodiments of liquid toner, a resin to pigment ratio can be from 1:1 to 10:1, and in other embodiments, from 4:1 to 8:1. Liquid toners are described further in Published U.S. Patent Applications 2002/0128349, entitled “Liquid Inks Comprising A Stable Organosol,” and 2002/0086916, entitled “Liquid Inks Comprising Treated Colorant Particles,” and U.S. Pat. No. 6,649,316, entitled “Phase Change Developer For Liquid Electrophotography,” all three of which are incorporated herein by reference.

Charge Transport Material

As described herein, an organophotoreceptor comprises a charge transport material having the formula

where X₁ and X₂ are, each independently, a linking group, such as —(CH₂)_(m)— group, where m is an integer between 1 and 20, inclusive, and one or more of the methylene groups is optionally replaced by O, S, N, C, B, Si, P, C═O, O═S═O, an NR_(a) group, a CR_(b) group, a CR_(c)R_(d) group, or a SiR_(e)R_(f) where R_(a), R_(b), R_(c), R_(d), R_(e), and R_(f) are, each independently, a bond, H, a hydroxyl group, a thiol group, a carboxyl group, an amino group, an alkyl group, an alkoxy group, an alkenyl group, an alkynyl group, a heterocyclic group, an aromatic group, or a part of a ring group, such as cycloalkyl groups or a benzo group;

Ar comprises an aromatic group;

R₁, R₂, and R₃ comprise, each independently, H, an alkyl group, an alkenyl group, an alkynyl group, an aromatic group, or a heterocyclic group; and

n is a distribution of integers between 1 and 100,000 with an average value of greater than one.

With respect to Formula (I), substitution is liberally allowed, especially on X₁, X₂, and Ar. Variation of the substituents, such as an aromatic group, an alkyl group, a heterocyclic group, and a ring group such as a benzo group, on on X₁, X₂, and Ar can result in various physical effects on the properties of the compounds, such as mobility, solubility, compatibility, stability, spectral absorbance, dispersibility, and the like, including, for example, substitutions known in the art to effect particular modifications.

In some embodiments, the organophotoreceptors as described herein can comprise an improved charge transport material of Formula (I) where X₁ is a —Y₄—CH₂— group, and X₂ is a —Y₅—CH₂CH(Y₆H)CH₂-Y₁-Z₁-Y₂-Z₂-Y₃—CH₂CH(Y₇H)— group where Y₁, Y₂, Y₃, Y₄, Y₅, Y₆, and Y₇ are, each independently, O, S, or NR where R is H, an alkyl group, an alkenyl group, an alkynyl group, an aromatic group, or a heterocyclic group; and Z₁ and Z₂, are, each independently, an aromatic group. In other embodiments, X₁ is a —O—CH₂— group, X₂ is a —OCH₂CH(OH)CH₂—Y₁-Z₁-Y₂-Z₂-Y₃—CH₂CH(OH)— group. In further embodiments, X₁ is a —O—CH₂— group and X₂ is a —OCH₂CH(OH)CH₂S—(C₆H₄)S(C₆H₄)SCH₂CH(OH)— group. In additional embodiments, Ar is an aromatic C₆H₃ group; and R₁ and R₂, each independently, comprise an aromatic group, such as an [(N,N-disubstituted)amino]aryl group. Specific, non-limiting examples of suitable charge transport materials within Formula (I) of the present invention have the following structures:

where n is a distribution of integers between 1 and 100,000 with an average value of greater than one.

Synthesis of Charge Transport Materials

The synthesis of the charge transport materials of this invention can be prepared by the following multi-step synthetic procedure, although other suitable synthetic procedures can be used by a person of ordinary skill in the art based on the disclosure herein.

The letter groups in the structures shown in the synthetic procedure below are defined as follows: X₁, X₂, X₃ and X₄ are, each independently, a linking group, such as a —(CH₂)_(m)— group, where m is an integer between 1 and 20, inclusive, and one or more of the methylene groups is optionally replaced by O, S, N, C, B, Si, P, C═O, O═S═O, an NR_(a) group, a CR_(b) group, a CR_(c)R_(d) group, or a SiR_(e)R_(f) where R_(a), R_(b), R_(c), R_(d), R_(e), and R_(f) are, each independently, a bond, H, a hydroxyl group, a thiol group, a carboxyl group, an amino group, an alkyl group, an alkoxy group, an alkenyl group, an alkynyl group, a heterocyclic group, an aromatic group, or a part of a ring group, such as cycloalkyl groups or a benzo group. Ar comprises an aromatic group. R₁, R₂, and R₃ comprise, each independently, H, an alkyl group, an alkenyl group, an alkynyl group, an aromatic group, or a heterocyclic group. The term n is a distribution of integers between 1 and 100,000 with an average value of greater than one. Q₁, Q₂, Q₃, and Q₄ are, each independently, a functional group, such as a hydroxyl group, a thiol group, a carboxyl group, and an amino group, that reacts with a reactive ring group. E₁ and E₂ are, each independently, a reactive ring group, such as an epoxy group, a thiiranyl group, an aziridino group, and an oxetanyl group.

The first step is the hydrazone formation reaction between an aldehyde or a ketone with an excess of hydrazine (H₂N—NH₂) to form the corresponding hydrazone of Formula (IV). The reaction can be catalyzed with acids, such as sulfuric acid and hydrochloric acid.

In the second step, the hydrazone of Formula (IV) may react with an aromatic aldehyde or ketone having two functional groups (Q₁ and Q₂ that can react with a reactive ring group) to form the corresponding azine compound of Formula (III). Q₁ and Q₂ can be the same or different. In some embodiments, Q₁ and Q₂, each independently, are selected from the group consisting of hydroxyl group, thiol group, carboxyl group, and an amino group. The reaction can be catalyzed with acids, such as sulfuric acid and hydrochloric acid.

In the third step, the azine compound of Formula (III) may react with at least an organic halide comprising one reactive ring group in the presence of an alkaline to form the corresponding azine compound of Formula (II) having two reactive ring groups (E₁ and E₂). E₁ and E₂ can be the same or different. If E₁ and E₂ are desired to be the same, one organic halide having one reactive ring group may be used. If E₁ and E₂ are desired to be different, the azine compound of Formula (III) may react with two different organic halides having one reactive ring group, either simultaneously or sequentially. The desired product may be isolated and purified by the convention purification techniques, such as column chromatography, thin layer chromatography, and recrystallization.

The reactive ring group may be selected from the group consisting of heterocyclic ring groups which have a higher strain energy than its corresponding open-ring structure. The conventional definition of strain energy is that it represents the difference in energy between the actual molecule and a completely strain-free molecule of the same constitution. More information about the origin of strain energy can be found in the article by Wiberg et al., “A Theoretical Analysis of Hydrocarbon Properties: II Additivity of Group Properties and the Origin of Strain Energy,” J. Am. Chem. Soc. 109, 985 (1987). The above article is incorporated herein by reference. The heterocyclic ring group may have 3, 4, 5, 7, 8, 9, 10, 11, or 12 members, in further embodiments 3, 4, 5, 7, or 8 members, in some embodiment 3, 4, or 8 members, and in additional embodiments 3 or 4 members. Non-limiting examples of such heterocyclic ring are cyclic ethers (e.g., epoxides and oxetane), cyclic amines (e.g., aziridine), cyclic sulfides (e.g., thiirane), cyclic amides (e.g., 2-azetidinone, 2-pyrrolidone, 2-piperidone, caprolactam, enantholactam, and capryllactam), N-carboxy-α-amino acid anhydrides, lactones, and cyclosiloxanes. The chemistry of the above heterocyclic rings is described in George Odian, “Principle of Polymerization,” second edition, Chapter 7, p. 508-552 (1981), incorporated herein by reference. In some embodiments of interest, E₁ and E₂, each independently, are selected from the group consisting of an epoxy group, a thiiranyl group, an aziridino group, and an oxetanyl group.

In some embodiments, at least one of the E₁ and E₂ groups is an epoxy group. Non-limiting examples of suitable organic halide comprising an epoxy group as the reactive ring group are epihalohydrins, such as epichlorohydrin. The organic halide comprising an epoxy group can also be prepared by the epoxidation reaction of the corresponding alkene having a halide group. Such epoxidation reaction is described in Carey et al., “Advanced Organic Chemistry, Part B: Reactions and Synthesis,” New York, 1983, pp. 494-498, incorporated herein by reference. The alkene having a halide group can be prepared by the Wittig reaction between a suitable aldehyde or keto compound and a suitable Wittig reagent. The Wittig and related reactions are described in Carey et al., “Advanced Organic Chemistry, Part B: Reactions and Synthesis,” New York, 1983, pp. 69-77, which is incorporated herein by reference.

The various preparation procedures of epoxy compounds have been disclosed in U.S. patent application Ser. Nos. 10/749,178, 10/634,164, 10/695,581, 10/663,970, and 10/692,389, and U.S. Provisional Patent Application Nos. 60/444,001 and 60/459,150. All the above application references are incorporated herein by reference.

In other embodiments, at least one of the E₁ and E₂ groups is a thiiranyl group. An epoxy compound, such as those described above, can be converted into the corresponding thiiranyl compound by refluxing the epoxy compound and ammonium thiocyanate in tetrahydrofuran. Alternatively, the corresponding thiiranyl compound may be obtained by passing a solution of the above-described epoxy compound through 3-(thiocyano)propyl-functionalized silica gel (commercially available form Aldrich, Milwaukee, Wis.). Alternatively, a thiiranyl compound may be obtained by the thia-Payne rearrangement of a corresponding epoxy compound. The thia-Payne rearrangement is described in Rayner, C. M. Synlett 1997, 11; Liu, Q. Y.; Marchington, A. P.; Rayner, C. M. Tetrahedron 1997, 53, 15729; Ibuka, T. Chem. Soc. Rev. 1998, 27, 145; and Rayner, C. M. Contemporary Organic Synthesis 1996, 3, 499. All the above four articles are incorporated herein by reference.

In further embodiments, at least one of the E₁ and E₂ groups is an aziridinyl group. An aziridine compound may be obtained by the aza-Payne rearrangement of a corresponding epoxy compound, such as one of those epoxy compounds described above. The thia-Payne rearrangement is described in Rayner, C. M. Synlett 1997, 11; Liu, Q. Y.; Marchington, A. P.; Rayner, C. M. Tetrahedron 1997, 53, 15729; and Ibuka, T. Chem. Soc. Rev. 1998, 27, 145. All the above three articles are incorporated herein by reference. Alternatively, an aziridine compound may be prepared by the addition reaction between a suitable nitrene compound and a suitable alkene. Such addition reaction is described in Carey et al., “Advanced Organic Chemistry, Part B: Reactions and Synthesis,” New York, 1983, pp. 446-448, incorporated herein by reference.

In additional embodiments, at least one of the E₁ and E₂ groups is an oxetanyl group. An oxetane compound may be prepared by the Paterno-Buchi reaction between a suitable carbonyl compound and a suitable alkene. The Paterno-Buchi reaction is described in Carey et al., “Advanced Organic Chemistry, Part B: Reactions and Synthesis,” New York, 1983, pp. 335-336, incorporated herein by reference.

The order of Steps 1-3 may be modified by a person of ordinary skill in the art based on the disclosure herein. For example, Step 1 and Step 2 may be reversed. In the other words, the aromatic aldehyde or ketone having two functional groups may react first with an excess of hydrazine to form the corresponding hydrazone, which then reacts with the aldehyde or ketone to form the corresponding azine of Formula (III). Another example is that Step 2 and Step 3 may be reversed so that the aromatic aldehyde or ketone having two reactive functional groups may react with an organic halide first and then with the hydrazone of Formula (IV) to form the corresponding azine of Formula (II).

The fourth step is the co-polymerization of the corresponding azine compound of Formula (II) having two reactive ring groups (E₁ and E₂) with a bridging compound (Q₃-Z-Q₄) in the presence of triethylamine to form the corresponding charge transport material of Formula (I). The bridging compound has a bridging group Z and at least two functional groups (Q₃ and Q₄), such as a hydroxyl group, a thiol group, an amino group, and a carboxyl group, that are reactive towards the reactive ring groups E₁ and E₂. Non-limiting examples of the bridging group Z include —(CH₂)_(k)— groups, where k is an integer between 1 and 30, inclusive, and one or more of the methylene groups is optionally replaced by O, S, N, C, B, Si, P, C═O, O═S═O, an NR_(m) group, a CR_(n) group, a CR_(o)R_(p) group, or a SiR_(q)R_(r) where R_(m), R_(n), R_(o), R_(p), R_(q), and R_(r) are, each independently, a bond, H, a hydroxyl group, a thiol group, a carboxyl group, an amino group, an alkyl group, an alkoxy group, an alkenyl group, an alkynyl group, a heterocyclic group, an aromatic group, or a part of a ring group, such as cycloalkyl groups or a benzo group.

The bridging compound may be a diol, a dithiol, a diamine, a dicarboxylic acid, a hydroxylamine, an amino acid, a hydroxyl acid, a thiol acid, a hydroxythiol, or a thioamine. Non-limiting examples of suitable dithiol are 4,4′-thiobisbenzenethiol, 1,4-benzenedithiol, 1,3-benzenedithiol, sulfonyl-bis(benzenethiol), 2,5-dimecapto-1,3,4-thiadiazole, 1,2-ethanedithiol, 1,3-propanedithiol, 1,4-butanedithiol, 1,5-pentanedithiol, and 1,6-hexanedithiol. Non-limiting examples of suitable diols are 2,2′-bi-7-naphtol, 1,4-dihydroxybenzene, 1,3-dihydroxybenzene, 10,10-bis(4-hydroxyphenyl)anthrone, 4,4′-sulfonyldiphenol, bisphenol, 4,4′-(9-fluorenylidene)diphenol, 1,10-decanediol, 1,5-pentanediol, diethylene glycol, 4,4′-(9-fluorenylidene)-bis(2-phenoxyethanol), bis(2-hydroxyethyl) terephthalate, bis[4-(2-hydroxyethoxy)phenyl] sulfone, hydroquinone-bis (2-hydroxyethyl)ether, and bis(2-hydroxyethyl) piperazine. Non-limiting examples of suitable diamine are diaminoarenes, and diaminoalkanes. Non-limiting examples of suitable dicarboxylic acid are phthalic acid, terephthalic acid, adipic acid, and 4,4′-biphenyldicarboxylic acid. Non-limiting examples of suitable hydroxylamine are p-aminophenol and fluoresceinamine. Non-limiting examples of suitable amino acid are 4-aminobutyric acid, phenylalanine, and 4-aminobenzoic acid. Non-limiting examples of suitable hydroxyl acid are salicylic acid, 4-hydroxybutyric acid, and 4-hydroxybenzoic acid. Non-limiting examples of suitable hydroxythiol are monothiohydroquinone and 4-mercapto-1-butanol. Non-limiting example of suitable thioamine is p-aminobenzenethiol. Non-limiting example of suitable thiol acid are 4-mercaptobenzoic acid and 4-mercaptobutyric acid. Almost all of the above bridging compounds are available commercially from Aldrich and other chemical suppliers.

X₁ and X₂ in Formula (I) are produced by the two ring-opening reactions between X₃-E₁ and Q₃, and between X₄-E₂ and Q₄. X₁ and X₂ may be parsed based on X₃, E₁, Q₃, X₄, E₂, and Q₄ in many different ways reasonable to a person skill in the art. A non-limiting example of reasonable parsing is that X₁ is X₃-E₁′-Q₃′ where E₁′-Q₃′ is the ring-opening reaction product between E₁ and Q₃; and X₂ is X₄-E₂′-Q₄′-Z where E₂′-Q₄′ is the ring-opening reaction product between E₂ and Q₄. Another non-limiting example of reasonable parsing is that X₁ is X₃; and X₂ is X₄-E₂′-Q₄′-Z-Q′₃-E′₁ where E₂′-Q₄′ is the ring-opening reaction product of E₂ and Q₄ and E₁′-Q₃′ is the ring-opening reaction product between E₁ and Q₃.

In some embodiments, Z comprises a Y₁-Z₁-Y₂-Z₂-Y₃— group where Y₁, Y₂, and Y₃ are, each independently, O, S, or NR where R is H, an alkyl group, an alkenyl group, an alkynyl group, an aromatic group, or a heterocyclic group; and Z₁, and Z₂, are, each independently, an aromatic group. In further embodiments, Y₁, Y₂, and Y₃ are, each independently, S; and Z₁, and Z₂, are, each independently, a phenylene group.

Although ring-opening reactions between reactive ring groups and functional groups that are reactive toward the reactive ring groups are disclosed in the fourth step above for the co-polymerization, other suitable synthetic reactions between two different functional groups that are reactive toward each other may be utilized for the co-polymerization in the fourth step. For example, Q₃ and Q₄ may be, each independently, hydroxyl or amine groups (or, each independently, carboxylic acid or halide groups) and E₁ and E₂ may be, each independently, carboxylic acid or halide groups (or, each independently, hydroxyl or amine groups) that react with the hydroxyl or amine groups (or carboxylic acid or halide groups) to produce a charge transport material of Formula (I) where X₁ and X₂ comprise, each independently, an ester or amide group respectively, or a combination thereof. Another example is that Q₃ and Q₄ may be amino groups (or carbonyl groups) and E₁ and E₂ may be carbonyl groups (or amino groups) that react with the amino groups (or carbonyl groups) to produce a charge transport material of Formula (I) where X₁ and X₂ comprise, each independently, an imine group. A further example is that Q₃ and Q₄ may be isocyanate groups (or, each independently, hydroxyl, thiol, or amine groups) and E₁ and E₂ may be, each independently, hydroxyl, thiol, or amine groups (or isocyanate groups) that react with the isocyanate groups (or hydroxyl, thiol, or amine groups) to produce a charge transport material of Formula (I) where X₁ and X₂ comprise, each independently, a urethane, thiocarbamate, or urea group respectively, or a combination thereof. Other suitable reactions can be used by a person of ordinary skill in the art based on the disclosure herein.

The invention will now be described further by way of the following examples.

EXAMPLES Example 1 Synthesis and Characterization Charge Transport Materials

This example describes the synthesis and characterization of Compound (1). The characterization involves chemical characterization of the compound. The electrostatic characterization, such as mobility and ionization potential, of the materials formed with the compound is presented in a subsequent example.

Bis(4,4′-diethylamino)benzophenone Hydrazone

A solution of bis(4,4′-diethylamino)benzophenone (108.1 g, 0.335 mol, from Aldrich), hydrazine monohydrate (98%, 244 ml, 5 mol, from Aldrich), and 10 ml of concentrated hydrochloric acid (from Aldrich) in 250 ml of 2-propanol was added to a 1000 ml 3-neck round bottom flask equipped with a reflux condenser and a mechanical stirrer. The solution was refluxed for approximately 6 hours with an intensive stirring until bis(4,4′-diethylamino)benzophenone disappeared. The solution was allowed to stand overnight. The crystals that formed upon standing were removed by filtration and then washed with 2-propanol to yield 79.2 g (70%) of bis(4,4′-diethylamino)benzophenone hydrazone. The melting point of the product was found to be 124-126° C. (recrystallized from 2-propanol). The ¹H-NMR spectrum (100 MHz) of the product in CDCl₃ was characterized by the following chemical shifts (δ, ppm): 7.37 (d, J=9.0 Hz, 2H, Ar); 7.14 (d, J=8.8 Hz, 2H, Ar); 6.74 (d, J=8.8 Hz, 2H, Ar); 6.58 (d, J=9.0 Hz, 2H, Ar); 4.85 (s, br, 2H, NH₂); 3.36 (m, 8H, N(CH ₂CH₃)₂; and 1.17 (m, 12H, N(CH₂CH ₃)₂). An elemental analysis yielded the following results in weight percent: C, 74.49; H, 7.68; N, 15.12, which compared with the following calculated values for C₂₁H₃₀N₄ in weight percent: C, 74.52; H, 8.93; N, 16.55.

4,4′-Bis(diethylamino)benzophenone 2,4-Dihydroxybenzaldehyde Azine

A mixture of bis(4,4′-diethylamino)benzophenone hydrazone (33.6 g, 0.1 mol, obtained previously), 2,4-dihydroxybenzaldehyde (15.1 g, 0.1 mol, obtained from Aldrich) and 40 ml of methanol was added to a 100 ml 3-neck round bottom flask equipped with a reflux condenser and a mechanical stirrer. The mixture was refluxed for 0.5 hour until one of the starting material disappeared. The solution was cooled to room temperature. The crystals formed in the cooled solution were filtered and washed repeatedly with methanol and dried at 50° C. in a vacuum oven for 5 hours to yield 36 g (79%) of crude product. The product was recrystallized from a mixture of dioxane and methanol in a volume ratio of 1:2. The product had a melting point of 205-206.5° C. The ¹H-NMR spectrum (100 MHz) of the product in CDCl₃ and two drops of DMSO-d₆ was characterized by the following chemical shifts (δ, ppm): 11.74 (s, 1H, OH); 9.44 (s, 1H, OH); 8.55 (s, 1H, N═CH); 7.51 (d, 2H, Ar); 7.35-6.98 (m, 3H, Ar); 6.80-6.48 (m, 4H, Ar); 6.48-6.18 (m, 2H, Ar); 3.35 (q, 8H, CH ₂CH₃, J=7.5 Hz); and 1.15 (m, 12H, CH₂CH ₃). An elemental analysis yielded the following results in weight percent: C, 73.11; H, 7.28; N, 12.10 which, compared with calculated values for C₂₈H₃₄N₄O₂ in weight percent of: C, 73.33; H 7.47; N 12.22.

4,4′-Bis(diethylamino)benzophenone 2,4-Bis(1,2-epoxypropoxy)benzaldehyde Azine

A mixture of 4,4′-bis(diethylamino)benzophenone 2,4-dihydroxybenzaldehyde_azine (33 g, 0.072 mol) and epichlorohydrin (85 ml, 1.1 mmol, commercially available from Aldrich) was stirred vigorously at 30-35° C. for 5 hours until the starting dihydroxy compound and its monosubstituted compound disappeared. During the 5-hour period, 12.2 g (0.22 mol) of powdered 85% potassium hydroxide and 3.6 g (28.8 mmol) of anhydrous sodium sulfate were added in three portions while the reaction mixture was kept at 20-25° C. After the termination of the reaction, the mixture was cooled to room temperature and filtered. The organic filtrate was treated with ethyl acetate and washed with distilled water until the washed water reached a neutral pH. Then, the organic layer was dried over anhydrous magnesium sulfate, treated with activated charcoal, and filtered. The solvents were removed and the residue was subjected to chromatography (silica gel, grade 62, 60-200 mesh, 150 Å, obtained from Aldrich) using a mixture of acetone and hexane in a volume ratio of 1:4 as the eluant. Fractions containing the product were collected and evaporated to yield 25.0 g (63%) of an oily residue. The oily residue crystallized after standing at room temperature for one month. The ¹H-NMR spectrum (100 MHz) of the product in CDCl₃ was characterized by the following chemical shifts (δ, ppm): 8.95 (s, 1H, CH═N); 8.02-7.18 (m, 5H, Ar); 6.80-6.32 (m, 6H, Ar); 4.44-3.64 (m, 6H, OCH₂CH); 3.39 (q, 8H, N(CH ₂CH₃)₂); 3.04-2.68 (m, 4H, CH₂ of oxirane); and 1.22 (m, 12H, N(CH₂CH ₃)₂). An elemental analysis yielded the following results in weight percent: C, 71.37; H, 7.28; and N, 9.65, which compared with calculated values for C₃₄H₄₂N₄O₄ in weight percent of: C, 71.55; H, 7.42; and N, 9.82.

Compound (1)

A mixture of 4,4′-bis(diethylamino)benzophenone 2,4-bis(1,2-epoxypro-poxy)benzaldehyde azine (1.61 g, 2.83 mmol), 4,4′-thiobisbenzenethiol (0.709 g, 2.8 mmol, from Aldrich, Milwaukee, Wis.) and triethylamine (0.2 ml, 1.415 mmol, from Aldrich, Milwaukee, Wis.) was refluxed in 20 ml of tetrahydrofuran (THF) under argon for 60 hours. The reaction mixture was cooled to room temperature and filtered through a layer of silica gel (3-4 cm thick, grade 62, 60-200 mesh, 150 Å) and the silica gel was washed with THF. The THF solution was concentrated to 15-20 ml by evaporation and then poured into 20-fold excess of methanol with intensive stirring. The resulted precipitate was filtered off and washed repeatedly with methanol and dried in a vacuum oven at 50° C. for 5 hours. The yield of the product was 1.4 g (60.4%).

Example 2 Charge Mobility Measurements

This example describes the measurement of charge mobility and ionization potential for charge transport materials, specifically Compound (1) above.

Sample 1

A mixture of 0.1 g of the Compound (1) and 0.1 g of polycarbonate Z was dissolved in 2 ml of tetrahydrofuran. The solution was coated on a polyester film with a conductive aluminum layer by a dip roller. After the coating was dried for 1 hour at 80° C., a clear 10 μm thick layer was formed. The hole mobility of the sample was measured and the results are presented in Table 1.

Mobility Measurements

Each sample was corona charged positively up to a surface potential U and illuminated with 2 ns long nitrogen laser light pulse. The hole mobility μ was determined as described in Kalade et al., “Investigation of charge carrier transfer in electrophotographic layers of chalkogenide glasses,” Proceeding IPCS 1994: The Physics and Chemistry of Imaging Systems, Rochester, N.Y., pp. 747-752, incorporated herein by reference. The hole mobility measurement was repeated with appropriate changes to the charging regime to charge the sample to different U values, which corresponded to different electric field strength inside the layer E. This dependence on electric field strength was approximated by the formula μ=μ₀e^(α√{square root over (E)}) Here E is electric field strength, μ₀ is the zero field mobility and α is Pool-Frenkel parameter. Table 1 lists the mobility characterizing parameters μ₀ and α values and the mobility value at the 6.4×10⁵ V/cm field strength as determined by these measurements for the four samples.

TABLE 1 Ionization μ₀ μ(cm²/V · s) α Potential Sample (cm²/V · s) at 6.4 · 10⁵ V/cm (cm/V)^(0.5) (eV) Sample 1/ 3.0 × 10⁻¹² 1.7 × 10⁻⁹ ~0.0079 5.42 Compound (1)

Example 3 Ionization Potential Measurements

This example describes the measurement of the ionization potential for the charge transport material described in Example 1.

To perform the ionization potential measurements, a thin layer of charge transport material about 0.5 μm thickness was coated from a solution of 2 mg of charge transport material in 0.2 ml of tetrahydrofuran on a 20 cm² substrate surface. The substrate was an aluminized polyester film coated with a 0.4 μm thick methylcellulose sub-layer.

Ionization potential was measured as described in Grigalevicius et al., “3,6-Di(N-diphenylamino)-9-phenylcarbazole and its methyl-substituted derivative as novel hole-transporting amorphous molecular materials,” Synthetic Metals 128 (2002), p. 127-131, incorporated herein by reference. In particular, each sample was illuminated with monochromatic light from the quartz monochromator with a deuterium lamp source. The power of the incident light beam was 2-5·10⁻⁸ W. A negative voltage of −300 V was supplied to the sample substrate. A counter-electrode with the 4.5×15 mm² slit for illumination was placed at 8 mm distance from the sample surface. The counter-electrode was connected to the input of a BK2-16 type electrometer, working in the open input regime, for the photocurrent measurement. A 10⁻¹⁵−10⁻¹² amp photocurrent was flowing in the circuit under illumination. The photocurrent, I, was strongly dependent on the incident light photon energy hν. The I^(0.5)=f(hv) dependence was plotted. Usually, the dependence of the square root of photocurrent on incident light quanta energy is well described by linear relationship near the threshold (see references “Ionization Potential of Organic Pigment Film by Atmospheric Photoelectron Emission Analysis,” Electrophotography, 28, Nr. 4, p. 364 (1989) by E. Miyamoto, Y. Yamaguchi, and M. Yokoyama; and “Photoemission in Solids,” Topics in Applied Physics, 26, 1-103 (1978) by M. Cordona and L. Ley, both of which are incorporated herein by reference). The linear part of this dependence was extrapolated to the hν axis, and the Ip value was determined as the photon energy at the interception point. The ionization potential measurement has an error of ±0.03 eV. The ionization potential value is given in Table 1 above.

As understood by those skilled in the art, additional substitution, variation among substituents, and alternative methods of synthesis and use may be practiced within the scope and intent of the present disclosure of the invention. The embodiments above are intended to be illustrative and not limiting. Additional embodiments are within the claims. Although the present invention has been described with reference to particular embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 

1. A charge transport material comprising a polymer having the formula

where X₁ and X₂ are, each independently, a linking group; Ar comprises an aromatic group; R₁, R₂, and R₃ comprise, each independently, H, an alkyl group, an alkenyl group, an alkynyl group, an aromatic group, or a heterocyclic group; and n is a distribution of integers between 1 and 100,000 with an average value of greater than one.
 2. A charge transport material according to claim 1 wherein R₁ and R₂ comprise, each independently, an [(N,N-disubstituted)amino]aryl group.
 3. A charge transport material according to claim 1 wherein X₁ and X₂, each independently, comprise a —(CH₂)_(m)— group, where m is an integer between 1 and 20, inclusive, and one or more of the methylene groups is optionally replaced by O, S, N, C, B, Si, P, C═O, O═S═O, an NR_(a) group, a CR_(b) group, a CR_(c)R_(d) group, or a SiR_(e)R_(f) where R_(a), R_(b), R_(c), R_(d), R_(e), and R_(f) are, each independently, a bond, H, a hydroxyl group, a thiol group, a carboxyl group, an amino group, an alkyl group, an alkoxy group, an alkenyl group, an alkynyl group, a heterocyclic group, an aromatic group, or a part of a ring group.
 4. A charge transport material according to claim 3 wherein X₁ is a —Y₄—CH₂— group, and X₂ is a —Y₅—CH₂CH(Y₆H)CH₂—Y₁-Z₁-Y₂-Z₂-Y₃—CH₂CH(Y₇H)— group where Y₁, Y₂, Y₃, Y₄, Y₅, Y₆, and Y₇ are, each independently, O, S, or NR where R is H, an alkyl group, an alkenyl group, an alkynyl group, an aromatic group, or a heterocyclic group; and Z₁ and Z₂, are, each independently, an aromatic group.
 5. A charge transport material according to claim 4 wherein Y₁, Y₂, and Y₃ are, each independently, S; and Z₁ and Z₂, are, each independently, a phenylene group.
 6. A charge transport material according to claim 1 wherein Ar is an aromatic C₆H₃ group.
 7. A method for forming a polymeric charge transport material, the method comprising the step of co-polymerizing a bridging compound having a bridging group and at least two functional groups with a charge transport material having the formula:

where X₃ and X₄ are, each independently, a linking group; Ar comprises an aromatic group; R₁, R₂, and R₃ comprise, each independently, H, an alkyl group, an alkenyl group, an alkynyl group, an aromatic group, or a heterocyclic group; and E₁ and E₂ are, each independently, a reactive ring group.
 8. A method for forming a polymeric charge transport material according to claim 7 wherein E₁ and E₂ are, each independently, an epoxy group, a thiiranyl group, an aziridino group, or an oxetanyl group.
 9. A method for forming a polymeric charge transport material according to claim 7 wherein X₃ and X₄ are, each independently, a — (CH₂)_(p)—group, where p is an integer between 1 and 10, inclusive, and one or more of the methylene groups is optionally replaced by O, S, N, C, B, Si, P, C═O, O═S═O, an NR_(g) group, a CR_(h) group, a CR_(i)R_(j) group, or a SiR_(k)R_(l) where R_(g), R_(h), R_(i), R_(j), R_(k), and R_(l) are, each independently, a bond, H, a hydroxyl group, a thiol group, a carboxyl group, an amino group, an alkyl group, an alkoxy group, an alkenyl group, an alkynyl group, a heterocyclic group, an aromatic group, or a part of a ring group.
 10. A method for forming a polymeric charge transport material according to claim 9 wherein X₃ and X₄, each independently, are O, S, or NR where R is H, an alkyl group, an alkenyl group, an alkynyl group, an aromatic group, or a heterocyclic group.
 11. A method for forming a polymeric charge transport material according to claim 7 wherein the at least two functional groups, each independently, are selected from the group consisting of a hydroxyl group, a thiol group, amino groups, and a carboxyl group.
 12. A method for forming a polymeric charge transport material according to claim 7 wherein the bridging group comprises a —(CH₂)_(k)— group, where k is an integer between 1 and 30, inclusive, and one or more of the methylene groups is optionally replaced by O, S, N, C, B, Si, P, C═O, O═S═O, an NR_(m) group, a CR_(n) group, a CR_(o)R_(p) group, or a SiR_(q)R_(r) where R_(m), R_(n), R_(o), R_(p), R_(q), and R_(r) are, each independently, a bond, H, a hydroxyl group, a thiol group, a carboxyl group, an amino group, an alkyl group, an alkoxy group, an alkenyl group, an alkynyl group, a heterocyclic group, an aromatic group, or a part of a ring group.
 13. A method for forming a polymeric charge transport material according to claim 7 wherein the bridging compound is selected from the group consisting of a diol, a dithiol, a diamine, a dicarboxlyic acid, a hydroxylamine, an amino acid, a hydroxyl acid, a thiol acid, a hydroxythiol, and a thioamine.
 14. A method for forming a polymeric charge transport material according to claim 7 wherein R₁ and R₂ comprise, each independently, an [(N,N-disubstituted)amino]aryl group. 